1. Field of the Invention
The present invention relates to a micro-strip antenna for use in a mobile communication apparatus, such as an airborne communication apparatus, a mobile telephone, or a cellular phone.
2. Description of the Related Art
A micro-strip antenna in which a radiation conductor is disposed on one main face of a dielectric substrate, and a ground conductor is disposed on the opposite main face of the substrate is compact, light, and thin. Therefore, such a micro-strip antenna is suitably used as an antenna member for use in a small-sized mobile communication apparatus, such as an airborne communication apparatus, a mobile telephone, or a cellular phone.
As shown in FIG. 7, a rectangular micro-strip antenna a includes a dielectric substrate b, a radiation conductor c formed on one main face of the substrate b, and a ground conductor d formed on the opposite main face of the substrate b. A through-hole e is formed in the dielectric substrate b and serves as a feed line to the radiation conductor c. Being energized via the through-hole e (feed point), the radiation conductor c radiates electromagnetic waves from its peripheral open ends. The thus-radiated electromagnetic waves are in the form of, for example, linearly polarized waves.
Reflection characteristics of the micro-strip antenna having the above structure vary greatly with input impedance. If input impedance fails to suitably match a 50.OMEGA. feed line, reflection characteristics will be degraded. As a result, the center frequency of a signal to be transmitted or received may deviate from the resonance frequency of the micro-strip antenna, potentially failing to efficiently transmit or receive electromagnetic waves.
For the reasons set forth above, a micro-strip antenna of the kind being considered here must employ means for matching its input impedance to the 50.OMEGA. feed line. Such a means is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 62-66703. According to this publication, a dielectric substrate is sandwiched between a radiation conductor b and a ground conductor c. A conductive plate is embedded in the dielectric substrate in parallel with the conductors b and c, and a feed line is electrically connected to the conductive plate and the ground conductor c. The conductive plate serves as a reactance compensation circuit element for changing the input impedance characteristics of the micro-strip antenna so as to suppress reflection characteristics in a predetermined band assigned to mobile communication apparatus and thus enabling implementation of a wide-band micro-strip antenna.
An important disadvantage of the above-described matching means is that the conductive plate must be embedded in the dielectric substrate, so that the resultant structure is relatively complex. As a consequence, fabrication of such micro-strip antennas is also relatively complex and difficult. Further, because the conductive plate is embedded in the dielectric substrate, the conductive plate cannot be adjusted from the outside.